This invention relates to a joint mechanism of a manipulator controlled by a spherical coordinate system and having an independent redundant degree of freedom, as used in a carrier device, etc.
Generally, it will be understood that a human hand connected to a wrist can be positioned by determining the length of the vector from shoulder to the wrist and the direction of that vector. Thus, a spherical coordinate system is conventionally used as the optimum coordinate system in describing such movements. In addition, the position of the elbow is related to the redundant degree of freedom which determines the posture of the arm, whereby man can control the posture of the arm to adapt to the working conditions and avoid an obstacle while working.
Conventionally, the prior art joint mechanism of a manipulator which performs such an actuating function has a construction such as shown in FIG. 1. This joint mechanism controls the right and leftward movement of the wrist W by operating a joint .alpha., it controls the up and downward movement of the wrist W by operating a joint .theta., and the length from the origin S to the wrist W is controlled by operating a joint .phi., to thereby position the wrist W. The posture of the arm is taken by changing the position of the elbow E by means of a joint .beta. located at the origin S of the coordinate system. The rotating shaft of the joint .alpha. is aligned with the Z-axis, and the X-axis and Y-axis are so provided that the X-Y coordinate plant includes the rotating shaft of the joint .beta.. Further, S' denotes an intersecting point of the rotating shafts of the joints .beta. and .theta..
In the prior art where the vector SW, which determines the position of the wrist W, is expressed by a spherical coordinate system, the elements l, .theta.' amd .alpha.' of the spherical coordinate system are not independent variables. As shown in FIG. 2, the length l of the vector SW is a function of the variables .phi. and .theta., its direction .theta.' being a function of the variables .theta., .phi. and .beta., and the other direction .alpha.' being a function of the variables .alpha., .theta., .phi. and .beta.. Therefore, the above elements of the coordinate system and the joints are not in relation of one-to-one so that when the joint .theta., for example, is operated, all of the elements l, .theta.' and .alpha.' may be varied. That is to say, this means that even if only one of the variables l, .theta.' and .alpha.' of the spherical coordinate system is desired to be varied, a complicated control is required. Further, when driving the joint .beta., which provides the redundant degree of freedom, the posture of the arm is varied, and the position of the wrist also may be changed at the same time.